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A First loophole-free Bell test?

  1. Aug 27, 2015 #1
    While this is the first Bell test that simultaneously addresses both the detection and the locality loophole, am I mistaken that this would still not be considered a loophole-free test?
    Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km
    http://arxiv.org/pdf/1508.05949v1.pdf

    “Spookiness” Confirmed by the First Loophole-free Quantum Test
    http://fqxi.org/community/forum/topic/2581
     
  2. jcsd
  3. Aug 27, 2015 #2

    gill1109

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    It also addresses the memory loophole. As far as I can tell it addresses every loophole which is of experimental nature, ie due to the experimenter not adhering to a rigorous experimental protocol. And we have known since Bell (1981) ("Bertlmann's socks") what that protocol is. No experiment can however escape metaphysical looholes, in particular, the super-determinism (aka conspiracy) loophole. The random settings were actually determined at the time of the big-bang so the stuff in Alice's lab already knows what settings Bob is going to use. One can only try to make invocation of that loophole ludicrous. Appeal to Occam's razor.
     
  4. Aug 27, 2015 #3

    DrChinese

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    And I see one of your papers was referenced as well. :smile:
     
  5. Aug 27, 2015 #4

    gill1109

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  6. Aug 27, 2015 #5

    stevendaryl

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    So when you say all loopholes, that includes:
    1. The possibility that the detections in EPR are not actually spacelike-separated.
    2. The possibility that detection/non-detection is not random, but may depend on device settings.
    3. The possibility that corresponding twin pairs are misidentified.
    4. ?
    (I don't know what the memory loophole is...)
     
  7. Aug 27, 2015 #6

    DrChinese

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    http://arxiv.org/abs/quant-ph/0205016

    Quantum nonlocality, Bell inequalities and the memory loophole
    Jonathan Barrett, Daniel Collins, Lucien Hardy, Adrian Kent, Sandu Popescu

    In the analysis of experiments designed to reveal violation of Bell-type inequalities, it is usually assumed that any hidden variables associated with the nth particle pair would be independent of measurement choices and outcomes for the first (n−1) pairs. Models which violate this assumption exploit what we call the {\it memory loophole}. We focus on the strongest type of violation, which uses the {\it 2-sided} memory loophole, in which the hidden variables for pair n can depend on the previous measurement choices and outcomes in both wings of the experiment. We show that the 2-sided memory loophole allows a systematic violation of the CHSH inequality when the data are analysed in the standard way, but cannot produce a violation if a CHSH expression depending linearly on the data is used. In the first case, the maximal CHSH violation becomes small as the number of particle pairs tested becomes large. Hence, although in principle the memory loophole implies a slight flaw in existing analyses of Bell experiments, the data still strongly confirm quantum mechanics against local hidden variables.
    We consider also a related loophole, the {\it simultaneous measurement loophole}, which applies if all measurements on each side are carried out simultaneously. We show that this can increase the probability of violating the linearised CHSH inequality as well as other Bell-type inequalities.
     
  8. Aug 27, 2015 #7

    gill1109

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    There is no detection/non-detection. There is a signal midway between Alice and Bob's laboratory that we have a pair of spins to measure. Measurement settings are chosen at random. The measurement is done, The settings and outcomes are stored. So quickly that Alice's diamond can't know Bob's settings. Read Bell (1981) "Bertlmann's socks".
     
  9. Aug 27, 2015 #8

    stevendaryl

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    I'm not sure that I understand your response. In the case of spin-1/2 EPR, if you have a source of twin pairs, it will occasionally happen that Alice detects one particle, but Bob fails to detect the corresponding antiparticle, or vice-versa. So you have to discard those cases when compiling your statistics.

    [edit]Not to mention the possibility that neither particle is detected.
     
    Last edited: Aug 27, 2015
  10. Aug 27, 2015 #9

    stevendaryl

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    This possible issue is listed in Wikipedia's list of loopholes
    https://en.wikipedia.org/wiki/Looph...ents#Detection_efficiency.2C_or_fair_sampling
     
  11. Aug 27, 2015 #10

    stevendaryl

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    From skimming the paper, it sounds like they avoid detection failure problems as follows:

     
  12. Aug 27, 2015 #11

    DrChinese

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    More of an issue with photons than electrons. With photons you also have the coincidence time window issue which is closely related. Such could account for a more appreciable portion of the total events. You don't really have that with electrons in these experiments.
     
  13. Aug 27, 2015 #12

    stevendaryl

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    Are you saying that every electron produced is eventually detected? (Or a sizable enough fraction of them?)
     
  14. Aug 27, 2015 #13
    This experiment has also got some press in Nature:
    Quantum 'spookiness' passes toughest test yet
    http://www.nature.com/news/quantum-spookiness-passes-toughest-test-yet-1.18255
     
  15. Aug 27, 2015 #14
    They clearly talk in the paper about the "free will" condition, locality loophole and the fair sampling loophole.

    What I can infer from this experiment is that previous Bell experimenters , had as much "free will" as a quantum random number generator.
     
  16. Aug 28, 2015 #15

    zonde

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    They don't produce electrons in this experiment. They say: "The boxes at location A and B each contain a single NV centre electron spin in diamond."
    So it's peace of diamond with "NV centre" (don't know what it is).
    So the "particles" are always there. However they are not always entangled. They produce photons from these NV centers and perform entanglement swapping. Bell state measurement rarely gives required output. But this measurement happens independently from basis selection and basis dependent measurement so that fair sampling loophole does not apply.
     
  17. Aug 28, 2015 #16

    gill1109

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    The experiment is performed exactly following the rigorous protocol of Bell (1981) "Bertlmann's socks". See figure 6 from that paper below. The "experimental unit" is not a pair of particles. Forget the word particle, forget the word wave. The experimental unit is a time-slot. At two distant locations, at the beginning of the time-slot, Alice and Bob each toss coins, switch a button on a machine according to H or T, and then observe an outcome +/-1.

    http://www.math.leidenuniv.nl/~gill/#loophole (look at pictures and follow link to http://www.math.leidenuniv.nl/~gill/loophole.txt)

    See also

    http://www.slideshare.net/gill1109/epidemiology-meets-quantum-statistics-causality-and-bells-theorem



    figure6.jpg
     
  18. Aug 28, 2015 #17

    f95toli

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    The experiments uses NV centres in diamond but the entanglement is still mediated by optical photons in a fibre. The reason NV centres are used in this type of experiment is that they can have very long coherence times and can be relatively easily be manipulated using microwave control pulses (NV centres have both optical and microwave transitions); they can also be read-out with very high fidelity.

    I saw a talk by one of the authors of this paper almost exactly a year ago. He was then very confident that they would be able to do this. I am glad he was right:smile:
     
  19. Aug 28, 2015 #18
    That is very interesting - even extremely interesting!
    Concerning "loophole-free", it very much depends on the kind of loopholes one considers. In particular, their claim that:

    "Our observation of a loophole-free Bell inequality violation thus rules out all local realist theories that accept that the number generators timely produce a free random bit and that the outputs are final once recorded in the electronics" ,

    can IMHO not be correct for the following reason. What is often overlooked or forgotten, is that an important possible theoretical loophole was created by Bell right at the start of his derivation. According to Jaynes (and I'm not aware that this was ever challenged), Bell's formula which was meant to just impose "no action at a distance", is technically wrong; unspecified additional assumptions are needed to make it right. Jaynes commented:

    "Thus while the QM formalism disagrees with Bell's factorization (14), it appears consistent with what we have called the "fundamentally correct" probability relations [..] . Recognizing this, it is evident that one could produce any number of experimental tests where the predictions of QM conflict with various predictions of (14)." - Jaynes, in "Clearing up mysteries - the original goal"

    As that discussion dates far back, it is unlikely that that loophole will ever be closed. Instead, and especially if indeed experimental loopholes are closed to the satisfaction of most, it could be the clue to better insight about "spookiness" which might be not that "spooky" after all.
     
    Last edited: Aug 28, 2015
  20. Aug 28, 2015 #19

    gill1109

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    Unfortunately Jaynes was wrong. He was flaggergasted when Stephen Gull gave an alternative proof of Bell's theorem showing using Fourier analysis that it was impossible to simulate the quantum correlations of the EPR-B experiment on a network of classical computers. http://arxiv.org/abs/1312.6403 http://www.mrao.cam.ac.uk/~steve/maxent2009/ http://www.mrao.cam.ac.uk/~steve/maxent2009/images/bell.pdf
    Basically, Jaynes had misunderstood the point of Bell's theorem and thought Bell was making an elementary probability mistake. He wasn't. Jaynes was often right, but not this time.
     
  21. Aug 28, 2015 #20
    That's very interesting, thanks! :smile:
    It will take me some time to absorb it of course...

    However, is that a formal proof? Why is it impossible to formally prove it by analytical derivation, as Bell attempted?
     
  22. Aug 28, 2015 #21

    gill1109

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    It is a formal proof. There are lots of formal proofs. The mathematical content is essentially trivial. You can prove it by Fourier analysis if you like, by logic and probability theory if you like, by calculus if you like. You can argue about the rigour and the generality of the different proofs. If you use calculus you are assuming calculus rules applies to physics, maybe you're wrong! I prefer an elementary proof by logic and discrete (counting) probability. Such as the proof I developed over the years which also allows to take account of *finite statistics* and *memory* and *time inhomogeneity*. Yet is as (in essence) as simple as they get. http://arxiv.org/abs/quant-ph/0110137 http://arxiv.org/abs/quant-ph/0301059 http://arxiv.org/abs/1207.5103 I'm very proud of the fact that the experimenters are now actually using my techniques to get their "paranoid" p-value (ie allowing any kind of dependence through the past, memory, etc etc ... just using the randomness in the measurement settings. Not assuming any kind of randomness in the physics).
     
  23. Aug 28, 2015 #22
    Thanks Richard - that's another thick paper to read!
    Concerning the hand-written proof by Gull, I notice the assumption "particle went into + channel (or - channel)"*. And you have "discrete counting probability" .... Could that fall in the category of what Bayes called "Bell theories" or is it really totally general? Can your class of models address the ("non-particulate"!) wave picture of reality as promoted by A. Neumaier (also a member of this forum), based on QFT? It's as yet unclear to me how to fill in your spreadsheet table based on such a picture.

    *PS. that contradicts the claim at the bottom of that hand-written pdf that "There are no physical assumptions"!
     
    Last edited: Aug 28, 2015
  24. Aug 28, 2015 #23

    gill1109

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    Bell (1981): "You might suspect that there is something specially peculiar about spin-1/2 particles. In fact there are many other ways of creating the troublesome correlations. So the following argument makes no reference to spin-1/2 particles, or any other particular particles.

    "Finally you might suspect that the very notion of particle, and particle orbit, freely used above in introducing the problem, has somehow led us astray. Indeed did not Einstein think that fields rather than particles are at the bottom of everything? So the following argument will not mention particles, nor indeed fields, nor any other particular picture of what goes on at the microscopic level. Nor will it involve any use of the words ‘quantum mechanical system’, which can have an unfortunate effect on the discussion. The difficulty is not created by any such picture or any such terminology. It is created by the predictions about the correlations in the visible outputs of certain conceivable experimental set-ups.

    "Consider the general experimental set-up of Fig. 7. To avoid inessential details it is represented just as a long box of unspecified equipment, with three inputs and three outputs. The outputs, above in the figure, can be three pieces of paper, each with either ‘yes’ or ‘no’ printed on it. The central input is just a ‘go’ signal which sets the experiment off at time t1. Shortly after that the central output says ‘yes’ or ‘no’. We are only interested in the ‘yes’s, which confirm that everything has got off to a good start (e.g., there are no ‘particles’ going in the wrong directions, and so on). At time t1 + T the other outputs appear, each with ‘yes’ or ‘no’ (depending for example on whether or not a signal has appeared on the ‘up’ side of a detecting screen behind a local Stern–Gerlach magnet). The apparatus then rests and recovers internally in preparation for a subsequent repetition of the experiment. But just before time t1 + T, say at time t1 + T – delta, signals a and b are injected at the two ends. (They might for example dictate that Stern–Gerlach magnets be rotated by angles a and b away from some standard position). We can arrange that c delta << L, where c is the velocity of light and L the length of the box; we would not then expect the signal at one end to have any influence on the output at the other, for lack of time, whatever hidden connections there might be between the two ends.

    "Sufficiently many repetitions of the experiment will allow tests of hypotheses about the joint conditional probability distribution P(A,B|a, b) of results A and B at the two ends for given signals a and b. Now of course it would be no surprise to find that the two results A and B are correlated, i.e., that P does not split into a product of independent factors: P(A,B|a,b) != P1(A|a)P2(B|b). But we will argue that certain particular correlations, realizable according to quantum mechanics, are locally inexplicable. They cannot be explained, that is to say, without action at a distance."
     
  25. Aug 28, 2015 #24

    Demystifier

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    There is no such thing as "loophole free test". A smart skeptic can always construct a new kind of loophole. If nothing else works, one can invoke a stupidity loophole; maybe nature is still local, but we are just too stupid to explain the correlations with a local theory.
     
  26. Aug 28, 2015 #25
    No, it suffices that Bell was smart enough to come up with his theorem. It shows that it is impossible to explain the correlations with a local theory, no matter your intelligence.
     
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